Low density insulation materials used in high temperature conditions are commonly made of fibrous ceramic materials. A typical material is SiO.sub.2 fibers used for outside tiles for aerospace vehicles. These materials withstand temperatures up to 1390.degree. C. and have a density of as low as 0.14 g/cm.sup.3. These materials are highly anisotropic because of the fibrous microstructure.
Ceramics have been made by spraying a solution or slurry of ceramic producing salts in a cryogenic liquid. In some instances the solvent is freeze-dried such that the ice is removed by sublimation in a vacuum to produce a ceramic powder. In other applications the frozen slurry is molded by freezing the slurry into the final shape.
The ceramic powder is formed into the desired shape and fired. This technique, as described in U.S. Pat. No. 3,516,935, produces a homogeneous ceramic body with more uniform structural characteristics than a fibrous ceramic. The density of the final ceramic product is close to theoretical or single crystal density.
Porous ceramics have been made by precipitating the salt, filtering the solution and freeze-drying the precipitate. The powder is a flaky-like particle which yields a ceramic with a porous but interlocked, irregular microstructure.